Both paths that you choose will see you with a much better gaming system, but I would really steer clear of buying another SAPPHIRE Radeon HD 7970 GHz Edition, and grabbing the new AMD Radeon R9 Fury X or NVIDIA GeForce GTX 980 Ti.

Both cards are going to be great for the 1080p monitor you have now, and just as good when you upgrade to a 2560x1440 display. The Fury X is a champion at 1440p gaming, sustaining 60FPS+ throughout our benchmarks at 2560x1440. I have no problems recommending you the single Fury X, as it'll beat HD 7970s in Crossfire without a problem, while consuming less power and generating less heat and noise.

Alternatively, you mentioned the GeForce GTX 980 Ti as an upgrade path. Switching to the GTX 980 Ti would provide the same performance jump, so the change is up to you really. Both the Fury X and GTX 980 Ti are incredibly powerful cards, both capable of maintaining 60FPS+ at High/Ultra details in all games right now at 1440p.